I would say yes and no, mostly yes. The only no being what you just recently said about does producing the highest number of offsrping when they're 3-5 yrs old. I always thought once a doe reach maturity she started to have less fawns on average (I could be remembering wrong). The part you said about fawns coming into estrous (they have the potential to reproduce once they hit 80 lbs), being the reason you haven't seen a decline in the length of your rut or fawning season was spot on, it makes perfect sense; this would also be another reason your maybe seeing lower fawn recruitment, b/c doe fawns will almost always give birth to 1 fawn. There's one thing you didn't mention that is good news for you based on the way you described your current herd. On average, does will typically give birth to the largest antlered offspring in their first few years of life (less than or equal to 3 yrs.).